Cloud storage is a system developed based on cloud computing that gathers numerous different types of storage devices in networks by application software to collaborate and collectively provide data storage and business access functions to the outside, through functionalities of cluster application, grid technology or distributed file system, etc. When operations and processing of a cloud computing system focus on storage and management of massive data, the cloud computing system needs to be configured with a lot of storage devices, and the cloud computing system becomes a cloud storage system. Therefore, the cloud storage is a cloud computing system focusing on data storage and management. To guarantee that a reliable cloud storage service can be provided to users, the provider of the cloud storage service generally backs up data of a production-end storage system by means of synchronous copying. When the production-end storage system has a failure or disaster, the backup-end storage system may be enabled to substitute for the production-end storage system so as to continue to provide data access to the host. The service provider is required to provide continuous data access services to users in a cloud storage environment.